In order to limit the global warming phenomenon, carbon dioxide (CO2) is extracted from combustion fumes in order to be sequestered in an underground reservoir.
Absorption methods using an aqueous amine solution to remove CO2 from a gas are commonly used. The gas is purified by contacting with the absorbent solution, then the absorbent solution is thermally regenerated.
One limitation of the absorption methods using an aqueous amine solution is the formation of degradation or side reaction products such as ammonia (NH3), some alcohols, ketones, organic acids. Acid degradation products (formic acid, acetic acid, etc.) react irreversibly with the amine and form non-regeneratable salts (heat stable salts). These acid products are thus trapped in the liquid phase in form of salts and periodically eliminated via solvent purification operations (by distillation or reclaiming for example). On the other hand, basic or aprotic degradation products do not react with the amine. They are therefore distributed between the liquid phase and the gas phase. They are generally present in small amounts but they can have a high vapour pressure. Non-zero amounts of such degradation products therefore escape into the decarbonated fumes.
The problem of degradation products entrained by decarbonated fumes can be solved using a water wash section downstream from the CO2 capture section. Document EP-502,596 can be mentioned by way of example, which provides a combustion fumes decarbonation method wherein the decarbonated fumes are washed with water to remove the reactive compounds from the absorbent solution, which were entrained by the fumes. However, water wash may be insufficient to reduce the proportion of degradation products to regulatory values.
As a supplement to water wash, document WO-2010/1,028,777 provides a second decarbonated fumes wash with an acid water so as to recover the degradation product traces that might not have been removed during the first water wash process.
The inventors have shown that the CO2 capture method using a water wash section as described in document EP-502,596 under standard operating conditions allows the discharge of degradation products in the fumes to be kept at the desired regulatory level. However, upon changes in the operating conditions of the method, such as stops, starts, sudden variations in temperature or in the composition of the fumes, the water wash system may be insufficient to limit discharges to the regulatory proportion. Although these degradation product discharge peaks are temporary, it is unthinkable not to abide by the required regulatory values. Furthermore, it would be uneconomic to dimension the water wash section in order to control the discharge peaks during these transient phases.
The present invention aims to monitor operating parameters of the method so as to be able to anticipate degradation product discharge variations, notably as regards ammonia, and to use, during such variation phases, an acid wash section of the type described in document WO-2010/1,028,777 in order to prevent degradation product discharge peaks that would exceed the allowed regulatory values.
Besides, in a preferred embodiment, the present invention aims to use the acid wash section in order to reduce the proportion of volatile degradation products, notably ammonia, contained in the absorbent solution without discharging them to the atmosphere.